Injection molding of many products is a practical, efficient and economical fabrication technique for many plastic articles and parts. Injection molding apparatus generally includes a mold formed of two portions, one of which is a cavity and the other a core which together correspond to the part being molded. The plastic or other material of which the article or part is fabricated is melted and mixed and then injected into the mold, following which the two portions of the mold are opened to eject the finished part referred to hereinafter as the workpiece. The equipment is designed to open and close the mold portions and of course they are held tightly together in appropriate alignment during the molding process. Usually one portion of the mold is held in fixed position while the other portion is movable to permit ejection of the workpiece.
The economies of injection molding are only achieved, however, if the volume of articles or parts is such to justify the set-up costs, including design and creation of the tooling. A change in any feature of a molded part must either be machined into what the mold is already producing, i.e, machined on the article itself rather than the mold, destroying much of the economies of injection molding, or the tool (mold) must be revised or replaced at substantial cost.
It is for that reason that replaceable cavity and core portions of molds have been developed. An excellent example of this is the apparatus of Taketa, U.S. Pat. No. 3,871,611 which contains a lucid disclosure on this technique. Further, the use of replaceable cavity and core portions of a mold is facilitated by the use of multiplate molds, which are also known in the art. Examples of multiplate molds are taught by Diehl, U.S. Pat. No. 3,892,512 and by Carlin, U.S. Pat. No. 3,327,355.
These and similar devices known in the art allow the costs of certain tooling changes to be reduced by replacement only of the core and cavity portions of the mold. Indeed, for some short run items, the core and cavity portions can be made of softer material that is easier and less expensive to machine, such as aluminum. This is because the core and cavity plates are reinforced by backing plates that may be made from more durable and rigid material such as steel.
Injection molding includes a requirement for effective removal of the workpiece from the mold cavity when the mold is opened so the mold can be recycled to produce a new workpiece. The apparatus and methods conventionally used in the injection molding art to eject the workpiece from the mold include knock-out pins and gas pressure applied directly to the workpiece. Knock-out pins have frequently involved mechanically moved or hydraulically pressurized operators. Of course, the knock-out pins must be properly positioned in regard to the cavity to achieve their purpose of removing the workpiece without damaging same. When multiplate molds or tools are used, with various different cavity locations, repeated relocation of the knock-out pins has historically necessitated drilling through the backing plates so often that they can begin to look like a swiss cheese and must be replaced. Furthermore, mechanically operated knock-out pins which are actuated using an ejector plate movable within an ejector box, must be located to avoid cooling water lines in the backing plate. Gas pressure or blow-out ejection devices suffer from the disadvantage that the orifice necessarily formed within the inner wall of the mold cavity frequently becomes plugged with a plastic or other material from which the workpiece is made. This difficulty with blow-out devices is well documented in the prior art herein described.
The present invention provides an apparatus that overcomes both of these foregoing disadvantages of the prior art by using a pneumatically actuated knock-out pin whose structure is entirely disposed within one or both molding plates. These are the portions of a multiplate mold that are retooled anyway for any design changes. Pneumatic channels within, but on the surface of, the molding plate bring the pressure source to wherever the knock-out pin should be disposed for best effectiveness.
Because a pneumatically pressurized knock-out pin utilizes a piston operator, a pre-examination search commissioned by the inventor of the present invention revealed a variety of prior art references dealing with knock-out devices, pneumatics, piston operators and the like. Most of this prior art is of modest relevance to the disclosure, but is recited for whatever background is obtainable therefrom.
For example, Schneider, U.S. Pat. No. 3,952,991 teaches a type of knock-out device that uses a piston as a valve rather than as the operator itself. It is really a type of blow-out device, and this reference also parenthetically discloses a use of a piston as a valve for flow of plastic or the like during the portion of the molding cycle when injection is not occurring. Another type of blow-out device is taught by Sheffield, et al., U.S. Pat. No. 4,653,997, which utilizes miltiple ports and a retractable port to assist removal of a lightweight workpiece such as a plastic cup. No pneumatically pressurized piston applies knock-out force to the workpiece in this reference.
Other pneumatically operated knock-out devices are those taught by Hujik, U.S. Pat. No. 3,914,086 which discloses a pneumatic piston operator with a custom shaped head for a particularized product. See also Dromigny, et al., U.S. Pat. No. 4,686,076 which uses a piston actuator in conjunction with an injection mold to hold a preprinted film in place in the mold to combine the film with the material from which the workpiece is fabricated.
Bearing in mind the foregoing, it is a principal object of the invention to provide an ejection apparatus for use with a multiplate mold, the structure of which is disposed entirely within a replaceable molding plate, usually the cavity plate.
A related object of the invention is to permit the ejection apparatus to be relocated as design changes are made without making changes to the backing plates or any other exterior surface of the mold.
A further object of the invention is to utilize a pneumatic operator for the ejection apparatus as an economical source of force that can be readily introduced into the mold and easily controlled from the mold exterior.
Another object of the invention is to utilize knock-out pins which can be machined to close tolerances with a bore in which they move axially to prevent leakage of the plastic or other molding material into the ejection apparatus.
An additional object of the invention is to utilize a compact apparatus for ejection so that as many knock-out pins can be positioned at optimal locations within a molding plate as are needed to optimize efficient ejection of the workpiece from the molding surface or cavity without damaging same.
An allied object of the invention is to use ejection apparatus of standardized configuration so that the apparatus will be interchangeable with a plurality of locations in each of a plurality of molding sites in a multicavity tool and also between a plurality of different molding plates.
A further object of the invention is to utilize an apparatus for ejection from an ejection molding multiplate tool that is appropriate for use with a molding plate that may be made from an easily machined material such as aluminum to economize on tooling changes for short run articles or parts.
Also an object of the invention is to provide a method for ejection of workpieces from a multiplate injection molding tool which can be repositioned economically as tooling changes are made within the molding plates.
A further object of the invention is to provide a method for ejection of a workpiece from a multiplate mold wherein repositioning can be achieved entirely within the molding plate.
A related object of the invention is to provide a method that facilitates repositioning of knock-out apparatus.
An additional object of the invention is to utilize a pressure (power) source external of the mold whose control and adjustment are easily and economically achieved.
A collateral object of the invention is to utilize a pressure source which, when introduced within the exterior surface of the mold will be harmless if the same leaks from the path to which it is directed.
An allied object of the invention is to utilize a pressure source which, when introduced into the mold can be directed without expensive fittings, piping, such as is necessary if the force were derived using hydraulic fluid to move an ejector plate as is known in the art.
Other objects and advantages of the present invention will be apparent to those skilled in the art upon reference to the following descriptions and the appended drawings.